Capacitance-type liquid level sensors measure the level of a liquid in a container and comprise a sensor probe submerged in a liquid medium. The sensor probe comprises a pair of electrodes that form a capacitor having the liquid and/or air between them acting as a dielectric. As the level of the liquid in the container varies the effective dielectric constant between the electrodes changes, which changes the capacitance of the sensor probe. The liquid level can be determined by detecting and measuring the change in capacitance.
It is known to use concentric, electrically conductive tubes as electrodes for sensor probes. This arrangement provides a relatively large surface area between the electrodes compared to the volume they occupy, allowing a relatively large capacitance to be formed. The larger the capacitance per unit height of the sensor probe the greater the change in capacitance per unit change in liquid level, allowing a greater sensitivity in level measurements. For concentric tube sensor probes, it is difficult to reduce the tolerance of the inner diameter of an outer tube and the outer diameter of an inner tube in a low cost liquid level sensor. Part to part variation in these dimensions leads to errors in level measurements. Custom spacers need to be developed that arrange the inner tube within the outer tube, leading to further increases in cost. For these and other reasons parallel plate-type sensor probes have been developed.
Parallel plate-type sensor probes comprise two flat plate electrodes that are spaced apart from each other. In mobile applications where the liquid level sensor is measuring the level of a fuel in a tank elongated probes can be susceptible to bending, sloshing of the fuel can cause deformation in the flat plate electrodes. Strength properties required to resist bending and deformation limit how much the thickness can be reduced and correlates directly to the cost of the sensor probe.
U.S. Pat. No. 4,512,067, issued Apr. 23, 1985 to Nissan Motor Company, Limited, the '067 patent, discloses a liquid container having electrode plates to detect liquid level by measuring capacitance. A sensor probe is formed by baffle plates 20A and 20B and electrode plates 30A, 30B and 30C. More specifically, a capacitor is formed between baffle plate 20A and electrode plate 30A, another capacitor is formed between baffle plate 20B and electrode plate 30B, and a third capacitor is formed between baffle plate 20B and 30C. The baffle plates serve dual functions, namely (1) suppressing a rapid and violent movement and/or heavy undulation of liquid fuel contained in a tank during operation of a vehicle, and (2) serving as electrodes for three capacitors. Although the thickness of baffle plates 20A and 20B is made smaller than the wall thickness of the upper and lower shells 12 and 14 of tank 10, there is a limit to how thin the baffle plates can be since they serve to suppress the rapid and violent movement and/or the heavy undulation of liquid fuel. Furthermore, the arrangement of the electrodes that make up the sensor probe capacitor is complex and costly and reduces the accuracy of liquid level measurements. In the event of sloshing, the oversized baffle plate electrodes redirect the flow of sloshing fuel in between the capacitor electrodes effectively changing the dielectric constant leading to level measurement errors.
The state of the art is lacking in sensor probes that provide accurate and repeatable capacitance measurements, occupy a relatively small volume, and are economical to manufacture. There is a need for an improved capacitance-type sensor probe with these characteristics.